Modeling Electric/Fossil Fuel Hybrid Boiler under ASHRAE 90.1-1999

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I am looking for help with regards to modeling a HVAC system under ASHRAE 90.1
1999. I am trying to determine which baseline HVAC system type to choose, as the
proposed model has a fossil fuel/electric hybrid boiler. Unlike 90.1 2007, where
the Baseline HVAC System Types include a fossil/electric hybrid boiler system,
it seems far more unclear under 1999. In Section 11.4.3 part k - (2) (page 59),
it says that

"For systems with mixed fuel heating sources, the system or systems that
use the secondary heating source type (the one with the smallest total installed
capacity for the spaces served by the system) shall be modeled identically in
the budget building design and the primary heating source type shall be
used in Figure 11.4.3 to determine budget system type."

However in the building that I am modeling, it has a 850 kW electric boiler
(lead boiler) with three further fossil fuel boilers with an installed capacity
of 3000 kW for backup. So my understanding of the code above is; even though the
electric boiler is the lead boiler in the proposed case, due to the fact that it
has a smaller installed capacity than the fossil fuel boilers, it should be
considered secondary, and thus the fossil fuel boilers will become the primary
heat source and so I should be using a fossil fuel boiler to heat my baseline
building. Does this seem correct? Also, I am unclear as to how I would be able
to model the electric boiler "identically in
the budget building design" as this is being
used to supply all of the heat to the heating loop in the proposed case.

Any help and ideas would be greatly appreciated.



Dónal OConnor's picture
Joined: 2011-10-02
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